The thymidine analogue Bromodeoxyuridine has been shown to alter the redundancy of the ribosomal RNA cistrons of Drosophila. We are using the drug to study the regulatory events influencing the function of these genes. Genetic, cytological, microhybridization, and cloning techniques are being used to determine in which tissues and stages the BrdU effect is manifested, whether sequences surrounding the rDNAs are altered in frequency, whether the BrdU effect is related to or differs from compensatory and abo related mechanism which also alter rDNA redundancy, and whether any of these mechanisms selectively discriminate intron bearing sequences of the rDNA. Long-range goals will include the isolation of gene products regulating the rDNA functions, and the identification of the specific nucleotide sequences to which they bind.